# How do I check that a device is online?

**URL:** <https://discourse.nodered.org/t/how-do-i-check-that-a-device-is-online/54189>\
**Category:** General\
**Created:** [24 November 2021 00:12 UTC](https://discourse.nodered.org/t/how-do-i-check-that-a-device-is-online/54189 "2021-11-24T00:12:49Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![Sprocket-N](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/sprocket-n/32/39067_2.png) [@Sprocket-N](https://discourse.nodered.org/u/Sprocket-N)\
**Post date:** [24 November 2021 00:12 UTC](https://discourse.nodered.org/t/how-do-i-check-that-a-device-is-online/54189/1 "2021-11-24T00:12:49Z")

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Hello,  
To control my outside lights and also monitor my greenhouse and garage temperatures.  
I have three ESP32 dev boards which communicate with Node-RED via a Mosquito server running on a Raspberry Pi4B. I have both manual and automated lighting control and I can see the temperatures for the past twenty-four hours on my dashboard. I'm pleased with my progress but I have come up against a problem I can't figure out.  
My WiFi is on its range limits and occasionally the one or more of the ESP32 dev boards go off-line. I have programmed a working reconnection for them in their respective C++ code, however I'd like to record the date and time one goes off line, so I can better understand the problem and decide if I need another WiFi range extender.  
To check if a controller is online every minute I send each one a message "Offline" on receiving this they reply "Online" using the same topic.  
On each controller's status topic I get a pair of messages "Offline" followed rapidly with "Online", provided the device is online. If not I only get the "Offline" message. I use this text on my Dashboard to display their status.  
How can I use these messages to trigger writing to a time stamped log file of when an ESP32 device goes off-line or comes on-line?

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<div class="post-metadata">

**Author:** ![UnborN](https://avatars.discourse-cdn.com/v4/letter/u/4491bb/32.png) [@UnborN](https://discourse.nodered.org/u/UnborN)\
**Post date:** [24 November 2021 01:11 UTC](https://discourse.nodered.org/t/how-do-i-check-that-a-device-is-online/54189/2 "2021-11-24T01:11:59Z")

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> [@Sprocket-N](#):
>
> How can I use these messages to trigger writing to a time stamped log file of when an ESP32 device goes off-line or comes on-line?

My approach would be to use the Ping node to check if the device is online / offline and also a Filter to allow messages only when the state has changed - not to flood the log and then write to the log file with the File write node.

**Example Flow :**

 ![image](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/3X/1/4/14be2508b086f792da3b9045c3aaddb82b1647d9.png)

```auto
[{"id":"c5a89e79ce9178c0","type":"ping","z":"54efb553244c241f","protocol":"Automatic","mode":"triggered","name":"","host":"","timer":"20","inputs":1,"x":470,"y":580,"wires":[["0333e185f5939315","65298e6fa7db0e87"]]},{"id":"394ab037e5188f3f","type":"inject","z":"54efb553244c241f","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payloadType":"date","x":170,"y":580,"wires":[["bfddde9ced331ae0"]]},{"id":"bfddde9ced331ae0","type":"function","z":"54efb553244c241f","name":"devices","func":"// expected output: 2011-10-05T14:48:00.000Z\nlet now = new Date().toLocaleString();\n\nmsg.payload =\n [\n {\n \"host\": \"192.168.0.66\",\n \"name\": \"Garden 1\",\n \"timeout\": 5000,\n \"datetime\": now\n },\n {\n \"host\": \"192.168.0.7\",\n \"name\": \"Garden 2\",\n \"timeout\": 5000,\n \"datetime\": now\n },\n {\n \"host\": \"192.168.0.17\",\n \"name\": \"ESP32\",\n \"timeout\": 5000,\n \"datetime\": now\n }\n]\n\nreturn msg;","outputs":1,"noerr":0,"initialize":"","finalize":"","libs":[],"x":320,"y":580,"wires":[["c5a89e79ce9178c0"]]},{"id":"0333e185f5939315","type":"debug","z":"54efb553244c241f","name":"1","active":false,"tosidebar":true,"console":false,"tostatus":false,"complete":"true","targetType":"full","statusVal":"","statusType":"auto","x":570,"y":520,"wires":[]},{"id":"b5079b376dea8da4","type":"function","z":"54efb553244c241f","name":"error","func":"// check for failed (false) pings\nif (msg.payload === false) {\n msg.payload = `${msg.ping.datetime} : Device ${msg.ping.name} Offline`\n}\nelse {\n msg.payload = `${msg.ping.datetime} : Device ${msg.ping.name} Online`\n}\n\nreturn msg;","outputs":1,"noerr":0,"initialize":"","finalize":"","libs":[],"x":850,"y":580,"wires":[["205d157d41f0a66c"]]},{"id":"205d157d41f0a66c","type":"file","z":"54efb553244c241f","name":"log","filename":"c:\\users\\user\\desktop\\ping_log.txt","appendNewline":true,"createDir":false,"overwriteFile":"false","encoding":"none","x":1030,"y":580,"wires":[[]]},{"id":"65298e6fa7db0e87","type":"rbe","z":"54efb553244c241f","name":"","func":"rbe","gap":"","start":"","inout":"out","septopics":true,"property":"payload","topi":"topic","x":650,"y":580,"wires":[["67232c5173a81985","b5079b376dea8da4"]]},{"id":"67232c5173a81985","type":"debug","z":"54efb553244c241f","name":"2","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"true","targetType":"full","statusVal":"","statusType":"auto","x":750,"y":520,"wires":[]}]

```

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<div class="post-metadata">

**Author:** ![gerry](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/gerry/32/21604_2.png) [@gerry](https://discourse.nodered.org/u/gerry)\
**Post date:** [24 November 2021 02:11 UTC](https://discourse.nodered.org/t/how-do-i-check-that-a-device-is-online/54189/3 "2021-11-24T02:11:22Z")

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MQTT has a LWT(last will testament) function. Provides info when MQTT disconnects. Check documentation.

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<div class="post-metadata">

**Author:** ![Sprocket-N](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/sprocket-n/32/39067_2.png) [@Sprocket-N](https://discourse.nodered.org/u/Sprocket-N)\
**Post date:** [27 November 2021 21:31 UTC](https://discourse.nodered.org/t/how-do-i-check-that-a-device-is-online/54189/4 "2021-11-27T21:31:38Z")

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Thank you both for replying,  
I'll try both suggestions, just to see if I can get them to work.

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<div class="post-metadata">

**Author:** ![system](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/1X/d073cd938eafa2e558d7c2cd59003b3ef4963033.png) [@system](https://discourse.nodered.org/u/system)\
**Post date:** [26 January 2022 21:31 UTC](https://discourse.nodered.org/t/how-do-i-check-that-a-device-is-online/54189/5 "2022-01-26T21:31:42Z")

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